Optical multilayer structure, optical switching device, and image display

ABSTRACT

An optical multilayer structure has a substrate, a light-absorbing first layer in contact with the substrate, a gap portion having a changeable size capable of causing an optical interference phenomenon, and a second layer. By changing the size of the gap portion, an amount of reflection, transmission, or absorption of incident light can be changed. For example, the substrate is made of carbon (C), the first layer is made of tantalum (Ta), and the second layer is made of silicon nitride (Si 3 N 4 ). Also in a visible light area, high response is realized. Consequently, the optical multilayer structure can be suitably used for an image display. The optical multilayer structure may be obtained by stacking, on a substrate made of a metal such as chromium (Cr), a first transparent layer made of a material having a high refractive index such as TiO 2  (n=2.40), a second transparent layer made of a material having a low refractive index such as MgF 2  (n=1.38), a gap portion having a changeable size capable of causing an optical interference phenomenon, and a third transparent layer made of a material having a high refractive index such as TiO 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical multilayer structure havinga function of reflecting, transmitting, or absorbing incident light, anoptical switching device using the same, and an image display using thesame.

2. Description of the Related Art

In recent years, importance of a display as a display device of videoinformation is increasing. As a device for the display and, further, asa device for optical communication, an optical memory device, an opticalprinter, and the like, development of an optical switching device (lightvalve) operating at high speed is in demand. Conventionally, as devicesof this kind, there are a device using a liquid crystal, a device usinga micro mirror (digital micro mirror device (DMD), trade mark of TexasInstruments), a device using a diffraction grating (Grating Light Valve(GLV™) of Silicon Light Machines (SLM)), and the like.

A GLV is obtained by fabricating a diffraction grating with an MEMS(Micro Electro Mechanical Systems) structure, and a high speed lightswitching device of 10 ns is realized with an electrostatic force. TheDMD performs switching of light by moving a mirror in the MEMSstructure. Although a display such as a projector can be realized byusing any of the devices, since the operation speed of each a liquidcrystal and the DMD is slow, to realize a display as a light valve, thedevices have to be arranged two-dimensionally, so that the structurebecomes complicated. On the other hand, the GLV is of a high speeddriving type. By scanning a one-dimensional array, a projection displaycan be realized.

However, the GLV has a diffraction grating structure and is thereforecomplicated, for example, since six devices have to be formed per pixeland diffracted rays emitted in two directions have to be converged toone by an optical system.

Light valves each of which can be realized with simple configuration aredisclosed in U.S. Pat. Nos. 5,589,974 and 5,500,761. The light valve hasa structure in which a translucent thin film having a refractive indexof {square root}{square root over ( )}n_(s) is provided on a substrate(having a refractive index of n_(s)) via a gap portion (gap layer). Inthis device, the thin film is driven by using an electrostatic force tochange the distance between the substrate and the thin film, that is,the size of the gap portion, thereby transmitting or reflecting a lightsignal. The refractive index of the thin film is {square root}{squareroot over ( )}n_(s) in contrast to the refractive index n_(s) of thesubstrate. It is said that, by satisfying such a relation, lightmodulation of high contrast can be carried out.

A device having the configuration as described above, however, has aproblem such that, when the refractive index n_(s) of the substrate isnot a large value like “4”, the light valve cannot be realized in avisible light region. From the viewpoint of the structure, desirably,the translucent thin film is made of, a material such as silicon nitride(Si₃N₄) (having refractive index n=2.0). The refractive index n_(s) ofthe substrate in this case is equal to four. In the visible lightregion, it is difficult to obtain such a transparent substrate and achoice of options of materials is narrow. At a wavelength forcommunication of infrared rays, the light valve can be realized by usinggermanium (Ge) (n=4). It seems difficult to apply the material for a useof a display in reality.

SUMMARY OF THE INVENTION

The invention has been achieved in consideration of the problems and afirst object of the invention is to provide an optical multilayerstructure having a simple configuration and a small structure, andhaving a variety of materials to be selected and realizing high responsealso in a visible light region, which can be suitably used for an imagedisplay or the like.

A second object of the invention is to provide an optical switchingdevice and an image display each using the optical multilayer structureand realizing high response.

A first optical multilayer structure according to the inventioncomprises a substrate, a light-absorbing first layer, a gap portionhaving a changeable size capable of causing an optical interferencephenomenon, and a second layer. Preferably, the first layer, the gapportion, and the second layer are stacked in accordance with this orderon the substrate.

In the first optical multilayer structure, when a complex index ofrefraction of the substrate is N_(s)(=n_(s)−i·k_(s), where n_(s) denotesa refractive index, k_(s) denotes an extinction coefficient, and irepresents an imaginary unit), a complex index of refraction of thefirst layer is N₁(=n₁−i·k₁, where n₁ denotes a refractive index, and k₁denotes an extinction coefficient), a refractive index of the secondlayer is n₂, and a refractive index of an incident medium is 1.0,preferably, the relation of the following Expression (1) is satisfied.{(n ₁−(n ₂ ²+1)/2)² +k ₁ ²−((n ₂ ²−1)/2)²}{(n _(s)−(n ₂ ²+1)/2)² +k _(s)²−((n ₂ ²−1)/2)²}<0  (1)

A first optical switching device according to the invention comprisesthe first optical multilayer structure of the invention, and drivingmeans for changing an optical size of the gap portion.

A first image display according to the invention is obtained byarranging a plurality of the first optical switching devices accordingto the invention one-dimensionally or two-dimensionally, for displayinga two-dimensional image by irradiating the optical switching device withlight of three primary colors and scanning the light by a scanner.

In the first optical multilayer structure according to the invention,the size of the gap portion is changed in a binary manner orcontinuously between an odd multiple of λ/4 (λ: design wavelength ofincident light) and an even multiple of λ/4 (including 0), therebychanging the amount of reflection, transmission, or absorption ofincident light in a binary manner or continuously.

When the first optical multilayer structure according to the inventionhas a configuration including no gap portion, a substrate, alight-absorbing first layer formed in contact with the substrate, and asecond layer formed in contact with a face of the first layer, on theside opposite to the substrate, it can be used as an antireflectionfilm.

The first optical switching device according to the invention switchesincident light when the optical size of the gap portion in the opticalmultilayer structure is changed by the driving means.

On the first image display according to the invention, when theplurality of optical switching devices arranged one-dimensionally ortwo-dimensionally with light, a two-dimensional image is displayed.

A second optical multilayer structure according to the inventioncomprises: a light-absorbing layer, portion, or substrate which does nottransmit incident light or a transparent substrate; a first transparentlayer made of a material having a low refractive index; and a secondtransparent layer made of a material having a high refractive index; thefirst and second transparent layers being stacked in accordance withthis order on the layer, portion, or substrate or the transparentsubstrate; and a gap portion having a changeable size capable of causingan optical interference phenomenon, provided between the light-absorbinglayer, portion, or substrate and the first transparent layer, or betweenthe first and second transparent layers. In the second opticalmultilayer structure, preferably, a refractive index nm and anextinction coefficient k_(m) (which is 0 in the case of the transparentsubstrate) of the light-absorbing layer, portion, or substrate or thetransparent substrate satisfy the relations of the following expressions(2) and (3), respectively.1≦n _(m)≦5.76  (2)k _(m)≦{square root}{square root over ( )}5.66−(n _(m)−3.38)²  (3)

In the specification, the “material having a high refractive index” is amaterial having a refractive index of 2.0 or higher such as TiO₂(n=2.4), Nb₂O₅ (n=2.1), or Ta₂O₅ (n=2.1). The “material having a lowrefractive index” is a material having a refractive index lower than 2.0such as MgF₂ (n=1.38), SiO₂ (n=1.46), or Al₂O₃ (n=1.67).

A third optical multilayer structure according to the inventioncomprises: a light-absorbing layer, portion, or substrate which does nottransmit incident light; a first transparent layer made of a materialhaving a high refractive index; a second transparent layer made of amaterial having a low refractive index; a third transparent layer madeof a material having a high refractive index; the first, second, andthird transparent layers being stacked in accordance with this order onthe layer, portion, or substrate, and a gap portion having a changeablesize capable of causing an optical interference phenomenon, providedbetween the light-absorbing layer, portion, or substrate and the firsttransparent layer, between the first and second transparent layers, orbetween the second and third transparent layers. In the second opticalmultilayer structure, preferably, a refractive index nm and anextinction coefficient k_(m) of the light-absorbing layer, portion, orsubstrate satisfy the relations of the following expressions (4) and(5), respectively, and do not satisfy the relations of theabove-described expressions (2) and (3), respectively.0.33≦n _(m)≦17.45  (4)k _(m)≦{square root}{square root over ( )}73.27−(n _(m)−8.89)²  (5)

A second optical switching device according to the invention comprisesthe second optical multilayer structure of the invention and drivingmeans for changing the optical size of the gap portion in the opticalmultilayer structure. A third optical switching device according to theinvention has the third optical multilayer structure of the inventionand driving means for changing the optical size of the gap portion inthe optical multilayer structure.

A second image display according to the invention is obtained byarranging a plurality of the second optical switching devices accordingto the invention one-dimensionally or two-dimensionally, for displayinga two-dimensional image when the plurality of the second opticalswitching devices are irradiated with light of three primary colors.

A third image display according to the invention is obtained byarranging a plurality of the third optical switching devices accordingto the invention one-dimensionally or two-dimensionally, for displayinga two-dimensional image when the plurality of the third opticalswitching devices are irradiated with light of three primary colors.

In the second optical multilayer structure according to the invention,the optical size of the gap portion between the first and secondtransparent films is changed in a binary manner or continuously betweenan odd multiple of λ/4 and an even multiple of λ/4 (including 0),thereby changing the amount of reflection of incident light entering aside opposite to the light-absorbing layer, portion, or substrate whichdoes not transmit incident light in a binary manner or continuously.

In the third optical multilayer structure according to the invention,the size of the gap portion between the first and second transparentlayers is changed in a binary manner or continuously between an oddmultiple of λ/4 and an even multiple of λ/4 (including 0), therebychanging the amount of reflection of incident light entering a sideopposite to the light-absorbing layer, portion, or substrate which doesnot transmit incident light in a binary manner or continuously.

In the second and third optical switching devices according to theinvention, the optical size of the gap portion in the optical multilayerstructure is changed by driving means, incident light is switched.

In the second and third image displays according to the invention, byirradiating the plurality of optical switching devices of the inventionarranged one-dimensionally or two-dimensionally with light, atwo-dimensional image is displayed.

A fourth optical switching device according to the invention comprises:a transparent substrate made of a non-metallic material; a firsttransparent layer in contact with the transparent substrate; a gapportion having a changeable size capable of causing an opticalinterference phenomenon; and a second transparent layer, the firsttransparent layer, the gap portion, and the second transparent layersbeing stacked in accordance with this order on the transparentsubstrate, wherein when a refractive index of the transparent substrateis n_(s), a refractive index of the first transparent layer is n₁, and arefractive index of the second transparent layer is n₂, the relation ofn_(s)<n₁ and the relation of n₁>n₂ are satisfied.

A fifth optical multilayer structure according to the inventioncomprises: a transparent substrate made of a non-metallic material; afirst transparent layer in contact with the transparent substrate; asecond transparent layer; a gap portion having a changeable size capableof causing an optical interference phenomenon; a third transparentlayer; and a fourth transparent layer. The first and second layers, thegap portion, and the third and fourth transparent layers are stacked inaccordance with this order on the transparent substrate. When arefractive index of the transparent substrate is n_(s), a refractiveindex of the first transparent layer is n₁, a refractive index of thesecond transparent layer is n₂, a refractive index of the thirdtransparent layer is n₃, and a refractive index of the fourthtransparent layer is n₄, the relation of n_(s)<n₁<n₂≈n₃ and the relationof n₄<n₁ are satisfied.

A fourth optical switching device according to the invention comprisesthe fourth optical multilayer structure of the invention and drivingmeans for changing the optical size of the gap portion in the opticalmultilayer structure. A fifth optical switching device according to theinvention has the fifth optical multilayer structure of the inventionand driving means for changing the optical size of the gap portion inthe optical multilayer structure.

A fourth image display according to the invention is obtained byarranging a plurality of fourth optical switching devices of theinvention one-dimensionally or two-dimensionally and displays atwo-dimensional image by irradiating the optical switching devices withlight of three primary colors, and scanning the light by a scanner.

A fifth image display according to the invention is obtained byarranging a plurality of the fifth optical switching devices of theinvention one-dimensionally or two-dimensionally and displays atwo-dimensional image by irradiating the optical switching devices withlight of three primary colors, and scanning the light by a scanner.

In the fourth and fifth optical multilayer structures according to theinvention, the size of the gap portion between the first and secondtransparent layers is switched between, for example, “λ/2” (λ:wavelength of incident light) and, preferably, “λ/4” and “0”, therebychanging the amount of reflection or transmission of incident lightentered from the transparent substrate side or a side opposite to thetransparent substrate. In the fourth optical multilayer structure, byswitching the size of the gap portion, an amount of reflection ortransmission largely changes at a specific wavelength range (singlewavelength). On the other hand, in the fifth optical multilayerstructure, the characteristic of reflection or transmission which isalmost uniform in a wide wavelength range is obtained.

In the fourth and fifth optical switching devices according to theinvention, the optical size of the gap portion in the optical multilayerstructure is changed by driving means, thereby switching incident light.

In the fourth and fifth image displays according to the invention, byirradiating a plurality of optical switching devices arrangedone-dimensionally or two-dimensionally with light, a two-dimensionalimage is displayed.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section showing the configuration in a case where agap portion in an optical multilayer structure according to a firstembodiment is “λ/4”.

FIG. 2 is a cross section showing the configuration in the case wherethe gap portion in the optical multilayer structure illustrated in FIG.1 “0”.

FIGS. 3A to 3D are cross sections for explaining a manufacturing processof the optical multilayer structure shown in FIG. 1.

FIGS. 4A to 4C are plan views for explaining a process subsequent to theprocess of FIGS. 3A to 3D.

FIGS. 5A and 5B are diagrams for explaining characteristics in the casewhere a gap portion in an optical multilayer structure using atransparent substrate and a transparent film is “0”.

FIGS. 6A and 6B are diagrams for explaining characteristics in the casewhere the gap portion of the optical multilayer structure using thetransparent substrate and the transparent film is “λ/4”.

FIG. 7 is an admittance diagram in the case where the substrate and afirst layer are made of a metal.

FIG. 8 is a diagram showing reflection characteristics of a concreteexample of the optical multilayer structure illustrated in FIG. 1.

FIG. 9 is a diagram for explaining optical admittance at the time of lowreflection in the example of FIG. 8.

FIG. 10 is a diagram for explaining optical admittance at the time ofhigh reflection in the example of FIG. 8.

FIG. 11 is a diagram showing reflection characteristics of anotherconcrete example of the optical multilayer structure illustrated in FIG.1.

FIG. 12 is a diagram for explaining optical admittance at the time oflow reflection in the example of FIG. 11.

FIG. 13 is a diagram for explaining optical admittance at the time ofhigh reflection in the example of FIG. 11.

FIG. 14 is an admittance diagram obtained by plotting optical admittanceof materials.

FIG. 15 is a diagram for explaining an example in which reflection canbe prevented even when optical admittance of the substrate and the firstlayer is on the inside of that of a second layer.

FIG. 16 is a cross section for explaining further another modificationof the first embodiment.

FIG. 17 is a cross section for explaining a driving method by staticelectricity of an optical multilayer structure.

FIG. 18 is a cross section for explaining another driving method bystatic electricity of the optical multilayer structure.

FIG. 19 is a cross section for explaining further another driving methodby static electricity of the optical multilayer structure.

FIGS. 20A and 20B are cross sections for explaining a driving method bymagnetic of the optical multilayer structure.

FIG. 21 is a diagram showing the configuration of an example of anoptical switching device.

FIG. 22 is a diagram showing the configuration of an example of adisplay.

FIG. 23 is a diagram showing another example of the display.

FIG. 24 is a diagram showing the configuration of a paper-state display.

FIG. 25 is a cross section showing the configuration when a gap layer inan optical multilayer structure according to a second embodiment is“λ/4”.

FIG. 26 is a cross section showing the configuration when the gap layerin the optical multilayer structure in FIG. 25 is “0”.

FIG. 27 is an admittance diagram in the case where the substrate is madeof a metal.

FIG. 28 is a cross section showing the configuration of an opticalmultilayer structure according to a third embodiment.

FIG. 29 is a cross section for explaining a modification of the thirdembodiment.

FIG. 30 is an admittance diagram for explaining the difference betweenan application range of the second embodiment and that of the thirdembodiment.

FIG. 31 is a diagram showing reflection characteristics of the opticalmultilayer structure illustrated in FIG. 25.

FIGS. 32A and 32B are diagrams for explaining optical admittance of theoptical multilayer structure shown in FIG. 25.

FIG. 33 is a diagram for explaining variations in reflectioncharacteristics according to positions of a gap layer in the secondembodiment.

FIG. 34 is a diagram for explaining the position of the gap layer incorrespondence with FIG. 33.

FIG. 35 is a diagram showing reflection characteristics of the opticalmultilayer structure illustrated in FIG. 28.

FIGS. 36A and 36B are diagrams for explaining optical admittance of theoptical multilayer structure shown in FIG. 28.

FIG. 37 is a diagram showing reflection characteristics of the opticalmultilayer structure illustrated in FIG. 29.

FIG. 38 is a diagram for explaining optical admittance of the opticalmultilayer structure shown in FIG. 29.

FIG. 39 is a diagram for explaining a modification of the secondembodiment.

FIG. 40 is a cross section showing the configuration when a gap layer inan optical multilayer structure according to a fourth embodiment is“λ/4”.

FIG. 41 is a cross section showing the configuration when the gap layerin the optical multilayer structure in FIG. 40 is “0”.

FIGS. 42A and 42B are diagrams showing reflection characteristics of theoptical multilayer structure illustrated in FIG. 40.

FIGS. 43A to 43C are diagrams for explaining the reflectioncharacteristics (optical admittance) of FIG. 42.

FIG. 44 is a cross section showing the configuration when a gap layer inan optical multilayer structure according to a fifth embodiment is“λ/4”.

FIG. 45 is a cross section showing the configuration when the gap layerin the optical multilayer structure in FIG. 44 is “0”.

FIG. 46 is a diagram showing reflection characteristics of the opticalmultilayer structure illustrated in FIG. 44.

FIGS. 47A and 47B are diagrams for explaining the reflectioncharacteristics (optical admittance) of the optical multilayer structureof FIG. 46.

FIG. 48 is a cross section for explaining a modification of the fourthembodiment.

FIG. 49 is a diagram showing reflection characteristics of the opticalmultilayer structure of FIG. 48.

FIG. 50 is a diagram showing reflection characteristics of the opticalmultilayer structure of FIG. 48.

FIG. 51 is a cross section for explaining another modification of thefourth embodiment.

FIG. 52 is a diagram showing (simulated) reflection characteristics ofthe optical multilayer structure of FIG. 51.

FIG. 53 is a diagram showing reflection characteristics (actualmeasurement values) of the optical multilayer structure of FIG. 51.

FIG. 54 is a diagram showing reflection characteristics (actualmeasurement values) of the optical multilayer structure of FIG. 51.

FIG. 55 is a diagram showing another reflection characteristics of theoptical multilayer structure in FIG. 48.

FIG. 56 is a diagram showing another reflection characteristics of theoptical multilayer structure in FIG. 48.

FIG. 57 is a cross section for explaining further another modificationof the fourth embodiment.

FIG. 58 is a diagram showing (simulated) reflection characteristics ofthe optical multilayer structure of FIG. 57.

FIG. 59 is a cross section for explaining further another modificationof the fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described in detail hereinbelow byreferring to the drawings.

First Embodiment

FIGS. 1 and 2 show a basic configuration of an optical multilayerstructure 1 according to a first embodiment of the invention. FIG. 1shows a high reflectance state where a gap portion 12 describedhereinlater in an optical multilayer structure 1 exists. FIG. 2 is a lowreflectance state where there is no gap portion 12 of the opticalmultilayer structure 1. The optical multilayer structure 1 is used as,concretely for example, an optical switching device. By arranging aplurality of optical switching devices one-dimensionally ortwo-dimensionally, an image display can be constructed. As will bedescribed in detail hereinlater, in the case of fixing the opticalmultilayer structure 1 in a structure as shown in FIG. 2, it can be usedas an antireflection film.

The optical multilayer structure 1 is constructed by stacking, on asubstrate 10, an optical-absorbing first layer 11 in contact with thesubstrate 10, the gap portion 12 having a changeable size capable ofcausing an optical interference phenomenon, and a second layer 13 inthis order.

When a complex index of refraction of the substrate 10 is N_(s)(=n_(s)−i·k_(s), where n_(s) denotes a refractive index, k_(s) denotesan extinction coefficient, and i represents an imaginary unit), acomplex index of refraction of the first layer 11 is N₁(=n_(i)−i·k₁,where n_(i) denotes a refractive index, k₁ denotes an extinctioncoefficient, and i represents an imaginary unit), a refractive index ofthe second layer 13 is n₂, and a refractive index of an incident mediumis 1.0 (air), they are set so as to satisfy the following relation ofEquation (6). The meaning will be described hereinlater.1≦n _(m)≦5.76  (6)

The substrate 10 may be made of an opaque, optical-absorbing material,e.g., a non-metal such as carbon (C) or graphite, a metal such astantalum (Ta), a metal oxide such as chromium oxide (CrO), a metalnitride such as titanium nitride (TiN_(x)), a carbide such as siliconcarbide (SiC), or a semiconductor such as silicon (Si), or may beobtained by forming a thin film made of an optical-absorbing material ona transparent substrate. The substrate 10 may be formed of a transparentmaterial such as glass or plastic or a translucent material having a lowextinction coefficient k.

The first layer 11 is an optical-absorbing layer and is made of a metalsuch as Ta, Ti, or Cr, a metal oxide such as CrO, a metal nitride suchas TiN_(x), a carbide such as SiC, a semiconductor such as silicon (Si),or the like.

The second layer 13 is made of a transparent material such as titaniumoxide (TiO₂) (n₂=2.4), silicon nitride (Si₃N₄) (n₂=2.0), zinc oxide(ZnO) (n₂=2.0), niobium oxide (Nb₂O₅) (n₂=2.2), tantalum oxide (Ta₂O₅)(n₂=2.1), silicon oxide (SiO) (n₂=2.0), stannic oxide (SnO₂) (n₂=2.0),ITO (Indium-Tin Oxide) (n₂=2.0), or the like.

Since the second layer 13 acts as a movable portion in a switchingoperation as will be described hereinlater, it is preferably made of astrong material having a high Young's modulus such as Si₃N₄. In the caseof driving the layer by static electricity, it is sufficient to includea transparent conductive film made of ITO or the like in a part of thesecond layer 13. Since the refractive index of Si₃N₄ and that of ITO areequal to each other, their thicknesses are arbitrary. When the firstlayer 11 and the second layer 13 are in contact with each other, it isdesirable to make the substrate side of the second layer 13 of Si₃N₄ andmake the incident medium side of ITO so as to prevent electricshort-circuiting at the time of contact.

A physical thickness d₁ of the first layer 11 is determined by thewavelength of incident light, values n and k of the material of thelayer, and optical constants of the substrate and the second layer 13and is, for example, a value in a range about 5 to 60 nm.

An optical thickness n₂·d₂ of the second layer 13 is equal to or smallerthan “λ/4” (λ denotes a design wavelength of incident light) in the casewhere the substrate 10 is made of a transparent material such as carbon,graphite, carbide, or glass and the first layer 11 is made of a materialhaving a high extinction coefficient k₁ such as tantalum (Ta). When thesubstrate 10 is made of a transparent material such as carbon, graphite,carbide, or glass and the first layer 11 is made of a material having alow extinction coefficient k₁ such as silicon (Si), an optical thicknessd₂ of the second layer 13 is larger than “λ/4” and smaller than “λ/2”for the following reason. Since the locus of optical admittance in thecase where the first layer 11 is made of Si moves upward in anadmittance diagram, an intersection between the first layer 11 and thesecond layer 12 is positioned upper than a real axis (positive side onan imaginary axis).

The film thicknesses d₁ and d₂ do not have to be strictly “λ/4” and“λ/2”, but may be about the values. For example, when one of the opticallayers is thicker than λ/4, it can be compensated by reducing thethickness of the other layer. When the refractive index is deviated fromthe equation (6) more or less, in some cases, it may be adjusted by thefilm thickness. In such a case, each of d₁ and d₂ is deviated from λ/4more or less. The above is also applied similarly to the otherembodiments. In the specification, the expression of “λ/4” includes“approximately λ/4”.

Each of the first and third layers 11 and 13 may take the form of acomposite layer constructed by two or more layers having opticalcharacteristics different from each other. In this case, the opticalcharacteristics (optical admittance) of the composite layer have to beequivalent to those in the case of a single layer.

The gap portion 12 is set to that its optical size (the interval betweenthe first layer 11 and the second layer 13) can be varied by drivingmeans described hereinlater. A medium filling the gap portion 12 may begas or liquid as long as it is transparent. Examples of the gas are air(having a refractive index n_(D)=1.0 with respect to sodium D ray (589.3nm)), nitrogen (N₂) (n_(D)=1.0) and the like. Examples of the liquid arewater (n_(D)=1.333) silicone oil (n_(D)=1.4 to 1.7), ethyl alcohol(n_(D)=1.3618), glycerin (n_(D)=1.4730), diiodomethane (n_(D)=1.737),and the like. The gap portion 12 may be in a vacuum state.

The optical size of the gap portion 12 changes in a binary manner orcontinuously between “an odd multiple of λ/4” and “an even multiple ofλ/4 (including 0)”. Accordingly, the amount of reflection, transmission,or absorption of incident light changes in a binary manner orcontinuously. Like the case of the first and second layers 11 and 13,even when the optical size is slightly deviated from a multiple of λ/4,it can be compensated by a slight change in the film thickness orrefractive index of the other layer. Consequently, the expression of“λ/4” includes “approximately λ/4”.

The optical multilayer structure 1 having the gap portion 12 can beformed by a manufacturing process shown in FIGS. 3A to 3D and FIGS. 4Ato 4C. First, as shown in FIG. 3A, on the substrate 10 made of carbon orthe like, the first layer 11 made of Ta is formed by, for example,sputtering. Subsequently, as shown in FIG. 3B, an amorphous silicon(a-Si) film 12 a is formed as sacrifice layer by, for example, CVD(Chemical Vapor Deposition). As shown in FIG. 3C, a photoresist film 14having a pattern of the shape of the gap portion 12 is formed. As shownin FIG. 3D, the photoresist film 14 is used as a mask and the amorphoussilicon (a-Si) film 12 a is selectively removed by, for example, RIE(Reactive Ion Etching).

After removing the photoresist film 14 as shown in FIG. 4A, the secondlayer 13 made of Si₃N₄ is formed by, for example, sputtering as shown inFIG. 4B. As shown in FIG. 4C, the amorphous silicon (a-Si) film 12 a isremoved by dry etching. In such a manner, the optical multilayerstructure 1 having the gap portion 12 can be manufactured.

In the optical multilayer structure 1, by changing the optical size ofthe gap portion 12 between an odd multiple of λ/4 and an even multipleof λ/4 (including 0) (for example, between “λ/4” and “0”) in a binarymanner or continuously, an amount of reflection, transmission, orabsorption of incident light is changed.

By referring to FIGS. 5A and 5B and FIGS. 6A and 6B, the meaning ofEquation (6) will be described.

The filter characteristic of the optical multilayer structure 1 asdescribed above can be explained by optical admittance. Opticaladmittance y is equal to a complex index N of refraction (=n−i·k, wheren denotes a refractive index, k denotes an extinction coefficient, and irepresents an imaginary unit). For example, the admittance of air isy(air)=1, and n(air)=1. The admittance of glass is y(glass)=1.52 andn(glass)=1.52.

When a transparent optical film is formed on a transparent substrate, onan optical admittance diagram as shown in FIG. 5B, a locus moves whiledrawing a circular arc as the film thickness increases. The lateral axisindicates the real axis (R_(e)) of the admittance and the vertical axisindicates an imaginary axis (I_(m)) of the admittance. For example, whenTiO₂ of n=y=2.40 or the like is formed on a glass substrate of n=y=1.52,the locus of composite optical admittance moves while drawing a circulararc from the point of y=1.52 as the film thickness increases. When theoptical film thickness of TiO₂ is λ/4, the locus of the compositeadmittance returns to the point of 2.42/1.52, that is, the point of 3.79(λ/4 law). This is the composite admittance in the case where the TiO₂film (first layer) having the thickness of λ/4 is formed on a glasssubstrate (transparent substrate). When the structure is seen fromabove, it is as if an integral substrate having n=3.79 is seen. Thereflectance R is calculated as 33.9% by the following expression (7) onthe interface with air.R=(n−1/n+1)²  (7)

When a film of, for example, n=y=1.947 is formed only by an optical filmthickness of λ/4 on the optical multilayer structure, on the opticaladmittance diagram, the locus moves clockwise from the point of 3.79.The composite admittance becomes Y=1.0 and is at the point of 1.0 on thereal axis. Specifically, it is equivalent that the compositeadmittance=composite refractive index is 1.0, that is, it becomesequivalent to that of air. Consequently, no reflection occurs on theinterface and the film can be regarded as what is called a V-coatanti-reflection film.

When the gap portion having n=1 (air) is provided only by the opticalfilm thickness of λ/4 on the TiO₂ film (n=2.4), the composite admittancebecomes Y₂=0.2638 as shown in FIGS. 6A and 6B. Further, when the film ofn=y=1.947 having the optical film thickness of λ/4 exists on the gapportion, the composite admittance becomes Y₃=14.37, and the locus ispositioned at the point of 14.37 on the real axis. The reflectance R atthis time is determined by substituting Y₃=14.37 into “n” in the aboveEquation 4 and is calculated as 76%. From the above, it is understoodthat the reflectance changes from “0%” to “76%” as the optical filmthickness of the gap portion (air layer) 12 is changed from “0” to“λ/4”.

The case where a transparent layer (TiO₂) which does not absorb light isformed on a substrate made of a transparent material such as glass hasbeen described above. In the complex index of refraction N=n−i·k (wheren denotes a refractive index, k denotes an extinction coefficient, and iindicates an imaginary unit), k is equal to zero in any of thematerials. In contrast, in the embodiment, at least the first layer 11out of the substrate 10 and the first layer 11 is made of anoptical-absorbing material such as an opaque metal material, metaloxide, or the like. That is, k is not equal to zero in the complex indexN₁ of refraction of the first layer 11. The features of the embodimentwill now be described hereinbelow.

FIG. 7 is an optical admittance diagram showing the locus passing a(1,0) point (air admittance) drawn by the second layer having therefractive index of n₂. The locus passes 1 and n₂ ² on the real axis anddraws a circular arc having a center of (n₂ ²+1)/2. In the case wherethe optical admittance (having a numerical value equal to the complexindex N of refraction) of the material of the substrate 10 exists withinthe circular arc, if the optical admittance of the material of the firstlayer 11 exists on the outside of the circular arc, composite opticaladmittance of the substrate 10 and the first layer 11 starts from apoint (indicated by N_(s) in the drawing) of the optical admittance ofthe substrate 10 and reaches a point (indicated by N₁ in the drawing) ofthe optical admittance of the first layer 11 while drawing a gentlecurve as the film thickness increases.

Since the optical admittance (equal to the complex index N ofrefraction) of the substrate 10 and the first layer 11 exits on bothsides of the circular arc drawn by the second layer 13, it surelycrosses the circular arc (point A). The thickness of the first layer 11is determined so that the composite admittance of the substrate 10 andthe first layer 11 arrives at the cross point A. From the cross point A,the composite admittance moves along the locus (circular arc) of thesecond layer 13.

When the second layer 13 is formed with a film thickness so that thecomposite admittance of the substrate 10, first layer 11, and secondlayer 13 is equal to one, reflection of incident light on the opticalmultilayer structure 1 becomes zero at a designed wavelength. When theoptical admittance of the substrate 10 and the first layer 11 exists onboth sides of the circular arc depending on the optical characteristicof the second layer 13, a combination of film thicknesses by whichreflection becomes zero certainly exists.

In this case, the optical admittance of the substrate 10 may exist onthe inside or outside of the circular arc. To satisfy such a condition,it is sufficient that the relation of optical constants of the materialsof the substrate 10 and the first layer 11 satisfies the followingexpression (8) or the expression (1) obtained by expressing theexpression (8) in another way when the complex index of refraction ofthe substrate 10 is N_(s)(=n_(s)−i·k_(s)), complex index of refractionof the first layer 11 is N₁(=n₁−i·k₁), refractive index of the secondlayer 13 is n₂, and refractive index of an incident medium is 1.0 (air).1≦n _(m)≦5.76  (8)

Consequently, in the case where the gap portion 12 of which size isvariable is provided between the first layer 11 and the second layer 13of the optical multilayer film constructed as described above, when theinterval d₃ is “0”, the structure serves as an antireflection film(refer to FIG. 2). When the interval d₃ is approximately equal to aquarter of the designed wavelength λ (λ/4), the structure serves as areflection film (refer to FIG. 1). That is, by varying the size of thegap portion 12 between “0” and “λ/4”, the optical switching device ofwhich reflectance can be changed between 0 and 70% or higher can berealized.

It is sufficient that the material of such an optical multilayerstructure 1 satisfies the condition as described above, and the choiceof options is wide. As for the configuration as well, since it issufficient to form three layers including the gap portion 12 on thesubstrate, manufacture is easy. Concrete examples will be describedhereinbelow.

CONCRETE EXAMPLES

FIG. 8 shows the relation between the wavelength (designed wavelength of550 nm) of incident light and reflectance in the case of using an opaquecarbon substrate (N_(s)=1.90, k=0.75) as the substrate, a Ta layer(N₁=2.46, k=1.90) as the first layer 11, an air layer (n=1.00) as thegap portion 12, and a stacked film (composite refractive index n₂=2.0,k=0) of an Si₃N₄ film and an ITO (Indium-Tin Oxide) film as the secondlayer 13. (a) indicates a characteristic in the case where an opticalfilm thickness of the gap portion (air layer) is “0” (low reflectanceside) and (b) indicates a characteristic in the case where the opticalfilm thickness is “λ/4” (137.5 nm) (high reflectance side). FIGS. 9 and10 are optical admittance diagrams shown as a reference. FIG. 9 showsthe case of low reflectance. FIG. 10 shows the case of high reflectance.

As obviously understood from FIG. 8, in the optical multilayer structure1 of the embodiment, when the optical film thickness of the gap portion(air layer) 12 is “λ/4”, a high reflection characteristic isdemonstrated. When the optical film thickness of the gap portion 12 is“0”, a low reflection characteristic is demonstrated. Specifically, whenthe optical film thickness of the gap portion 12 is switched between anodd multiple of “λ/4” and an even multiple of “λ/4” (including 0″, thehigh reflection characteristic and the low reflection characteristic arealternately displayed.

In the case of using a metal film (such as Ta, having k₁=1.90) of a highextinction coefficient k₁ as the first layer 11, the optical filmthickness of the second layer 13 becomes “λ/4”. In the case of using asemiconductor material having a low k₁ as the first layer (for example,Si having k₁=0.63), the optical film thickness of the second layer 13 islarger than “λ/4” (yet smaller than λ/2). FIG. 11 shows the reflectioncharacteristic (designed wavelength of 550 nm) in the case of formingthe substrate 10 of graphite (refractive index n_(s)=1.90, k=0.75),making the first layer 11 of silicon (having refractive index n₁=4.40,k=0.63, and film thickness 13.09 nm), and making the second layer 13 bya stacked film (composite refractive index n₂=2.0, k=0, and filmthickness=83.21 nm) of an Si₃N₄ film and an ITO (Indium-Tin Oxide) film.(a) indicates a characteristic in the case where an optical filmthickness of the gap portion (air layer) is “0” (low reflectance side)and (b) indicates a characteristic in the case where the optical filmthickness is “λ/4” (137.5 nm) (high reflectance side). FIGS. 12 and 13are optical admittance diagrams in this case. FIG. 12 shows the case oflow reflectance. FIG. 13 shows the case of high reflectance.

In the two examples, it is assumed that the substrate 10 is made ofopaque carbon or graphite. Carbon and graphite are suitable as thematerials of the substrate 10 since the optical admittance (equal to thecomplex index of refraction) of each of them is positioned on the insideof the circular locus drawn so as to pass (1,0) of a transparent filmhaving a refractive index of 2.0 on the admittance diagram for thereason that the optical admittances of many metal materials are placedon the outside of the circle.

For reference, FIG. 14 is an admittance diagram obtained by plotting theoptical admittances of some materials. FIG. 14 also shows loci passing a(1,0) point (air admittance) drawn by the material having the refractiveindex of n=2 and the TiO₂ (n=2.4). When the substrate 10 is made of anyof the materials in the circle, the first layer 11 is made of any of thematerials outside of the circle, and the second layer 13 is made of anyof the materials on the circle, a combination of film thicknessesrealizing low reflectance (almost zero) surely exists. For example, whenthe substrate 10 is made of carbon (indicated by C in the drawing), thefirst layer 11 is made of any of the materials outside of the circlehaving n=2 (most of the materials in the drawings), and the second layer13 is made of any of the materials having n=2 (such as Si₃N₄, ITO, ZnO,or the like), an optical switching device having excellentcharacteristics can be realized.

When TiO₂ is used as the second layer 13, the material of the substrate10 is selected from silicon (Si), carbon (C), tantalum (Ta), germanium(Ge) film, graphite, glass, or the like, and the material of the firstlayer 11 is selected from the other metals in the diagram. In such amanner, an optical switching device having excellent characteristics canbe realized.

In FIG. 14, representative metal materials, semiconductor, and the likeare plotted. It is also possible to plot other materials and determinewhether they are on the inside or outside of the circle to therebyeasily select the materials of a good combination.

It is a sufficient condition to realize the optical structure having anexcellent characteristic but is not a necessary condition that theoptical characteristics of the substrate 10 and the first layer 11 existon the inside and outside of the circle of the second layer 13 asdescribed above for the following reason. The locus of the compositeoptical admittance to form a light absorbing film (that is, k≠0) on thesubstrate 10 does not extend from the admittance of the substrate 10toward the optical admittance of a film forming material linearly butextends toward the optical admittance of a film forming material whilelargely curving. Consequently, when the degree of curve is large, evenif there is the optical admittance of the first layer 11 on the insideof the circle of the second layer 13, the composite optical admittancemay cross the circle of the second layer 13.

FIG. 15 is a diagram showing such an example. When graphite is appliedto form the first layer 11 on the substrate 10 made of carbon C, theoptical admittance of the first layer 11 curves and crosses the circulararc of n=2 twice. By setting the film thickness so that the carbon C isswitched to the film of n=2 (such as Si₃N₄, ITO, ZnO, or the like) atone of the points, an optical multilayer structure 1 having an excellentcharacteristic can be realized.

In the embodiment, in a visible light region of, for example, 550 nm,the low reflectance can be set to almost zero and the high reflectancecan be set to 70% or higher. Consequently, modulation of high contrastcan be performed. Moreover, since the configuration is simple, thestructure can be fabricated more easily as compared with a gratingdiffraction structure such as GLV and a complicated three-dimensionalstructure such as a DMD. Although six lattice-state ribbons arenecessary for one pixel in the GLV, one ribbon is sufficient per pixelin the optical multiplayer structure 1 of the embodiment. Thus, theconfiguration is simple and a small structure can be fabricated. Since amovable portion moving range is at most “λ/2”, high response at thelevel of 10 ns can be realized. In the case of using the structure as alight valve for a display, the structure can be realized by a simpleconfiguration of a one-dimensional array as will be described.

Further, since the optical multilayer structure 1 of the embodiment isessentially different from a narrow-bandpass filter having a structurein which a gap portion is sandwiched by a metal thin film and areflection layer, that is, a Fabry-Perot type filter, the bandwidth of alow reflection band can be made wide. Therefore, a relatively widemargin for managing the film thickness at the time of manufacture can beobtained, so that design flexibility is increased.

In the embodiment, the refractive index of each of the substrate 10 andthe first layer 11 may be an arbitrary value in a certain range, so thatthe choice of options in selecting materials is widened. When thesubstrate 10 is made of an opaque material, incident light is absorbedby the substrate 10 at the time of low reflectance, and there isconsequently no fear that stray light or the like is generated.

By using the optical multilayer structure 1 of the embodiment asdescribed above, a high speed, small optical switching device and animage display can be realized. The details will be describedhereinlater.

Although the gap portion in the optical multilayer structure 1 is asingle layer in the foregoing embodiment, it can consists of a pluralityof layers, for example, two layers as shown in FIG. 16. In a structureof FIG. 16, the first layer 11, first gap portion 12, second layer 13, asecond gap portion 30, and a third transparent layer 31 are sequentiallystacked on the substrate 10, and the second layer 13 and the thirdtransparent layer 31 are supported by, for example, supporting members15 and 32 made of silicon nitride, respectively.

In the optical multilayer structure, the second layer 13 as anintermediate layer is displaced in the vertical direction, one of thefirst and second gap portions 12 and 30 is accordingly narrowed, and theother gap portion is widened, thereby changing the reflectioncharacteristic.

Driving Method

Concrete means for changing the size of the gap portion 12 in theoptical multilayer structure 1 will now be described.

FIG. 17 shows an example of driving an optical multilayer structure byan electrostatic force. In the optical multilayer structure, electrodes16 a and 16 a made of aluminum or the like are provided on the firstlayer 11 on the transparent substrate 10, the second layer 13 issupported by the supporting member 15 made of silicon nitride (Si₃N₄),and electrodes 16 b, 16 b are formed in positions facing the electrodes16 a, 16 a of the supporting member 15.

In the optical multilayer structure, by an electrostatic force generatedby a potential difference which occurs when a voltage is applied to theelectrodes 16 a and 16 a and the electrodes 16 b and 16 b, the opticalfilm thickness of the gap portion 12 is switched in binary, for example,between “λ/4” and “0” or between “λ/4” and “λ/2”. Obviously, bycontinuously changing a voltage applied to the electrodes 16 a and 16 aand the electrodes 16 b and 16 b, the size of the gap portion 12 can becontinuously changed in a range of certain values, and an amount ofreflection, transmission, absorption, or the like of incident light ischanged continuously (in an analog manner).

The optical multilayer structure can be driven by an electrostatic forceby other methods shown in FIGS. 18 and 19. In the optical multilayerstructure 1 shown in FIG. 1, a transparent conductive film 17 a made of,for example, ITO (Indium-Tin Oxide) is provided on the first layer 11 onthe transparent substrate 10, a second layer 13 made of, for example,SiO₂ is formed in a bridge structure, and a transparent conductive film17 b made of ITO is provided on the outer face of the second layer 13.

In the optical multilayer structure, the optical film thickness of thegap portion 12 can be switched by an electrostatic force generated by apotential difference which occurs when a voltage is applied across thetransparent conductive films 17 a and 17 b.

In the optical multilayer structure shown in FIG. 19, in place of thetransparent conductive film 17 a in the optical multilayer structure ofFIG. 18, for example, a tantalum (Ta) film is provided as the firstlayer 11 having conductivity.

To drive the optical multilayer structure, other than the electrostaticforce, various methods such as a method of using a micromachine such asa toggle mechanism or piezoelectric device, a method of using a magneticforce, and a method of using a shape memory alloy. FIGS. 20A and 20Bshow modes of driving an optical multilayer structure by using amagnetic force. In this optical multilayer structure, a magnetic layer40 made of a magnetic material such as cobalt (Co) having an opening isprovided on the second layer 13, and an electromagnetic coil 41 isprovided below the substrate 10. By switching the turn-off and turn-onof the electromagnetic coil 41, the size of the gap portion 12 isswitched between, for example, “λ/4” (FIG. 20A) and “0” (FIG. 20B),thereby changing the reflectance.

Optical Switching Apparatus

FIG. 21 shows the configuration of an optical switching device 100 usingthe optical multilayer structure 1. The optical switching device 100 isobtained by arranging a plurality of (four in the diagram) opticalswitching devices 100A to 100D on a substrate 101 made of carbon or thelike in a one-dimensional array. The optical switching devices 100A to100D may be also arranged two-dimensionally. In the optical switchingdevice 100, along one direction (device arrangement direction) of thesurface of the substrate 101, for example, a Ta film 102 is formed.

On the substrate 101, a plurality of Si₃N₄ films 105 are disposed in adirection orthogonal to the Ta film 102. On the outside of the Si₃N₄film 105, an ITO film 106 as a transparent conductive film is formed.The ITO film 106 and the Si₃N₄ film 105 correspond to the second layer13 in the embodiment and form a bridge structure in a position over theTa film 102. Between the Ta film 102 and the ITO film 106, a gap portion104 of which size changes according to a switching (on/off) operation.An optical film thick of the gap portion 104 changes between, forexample, “λ/4” (137.5 nm) and “0” at a wavelength (λ=550 nm) of incidentlight.

The optical switching devices 100A to 100D switch the optical filmthickness of the gap portion 104 between, for example, “λ/4” and “0” byan electrostatic force generated by a potential difference caused by avoltage applied to the Ta film 102 and the ITO film 106. FIG. 21 shows astate where gap portion 104 in each of the optical switching devices100A and 100C is “0” (that is, low reflectance state) and a state wherethe gap portion 104 in each of the optical switching devices 100B and100D is “λ/4” (that is, high reflectance state). By the Ta film 102, theITO film 106, and a voltage applying device (not shown), “driving means”of the invention is constructed.

In the optical switching device 100, when the Ta film 102 is grounded toset the potential at 0V, and +12V is applied to the ITO film 106 formedon the second layer side, by the potential difference, an electrostaticforce is generated between the Ta film 102 and the ITO film 106. In FIG.21, like the optical switching devices 100A and 100C, the first andsecond layers are closely attached to each other and the gap portion 104is in the “0” state. In this state, incident light P₁ passes through themultilayer structure and is absorbed by the substrate 21.

When the transparent conductive film 106 on the second layer side isgrounded to set the potential at 0V, there is no electrostatic forcebetween the Ta film 102 and the ITO film 106. In FIG. 21, like theoptical switching devices 100B and 100D, the first and second layers areapart from each other and the gap portion 12 enters the “λ/4” state. Inthis state, the incident light P₁ is reflected as reflection light P₃.

In such a manner, according to the embodiment, in each of the opticalswitching devices 100A to 100D, the gap portion is switched in binary byan electrostatic force, thereby switching the incident light P₁ betweenthe state where there is no reflection light and the state where thereflection light P₃ is generated in binary. Obviously, by continuouslychanging the size of the gap portion as described above, the incidentlight P₁ can be continuously changed between the state where there isnot reflection light to the state where the reflection light P₃ isgenerated.

In each of the optical switching devices 100A to 100D, the distance ofmovement of the movable portion is at most about “λ/2 (or λ/4)” ofincident light. Consequently, response is about 10 ns and sufficientlyhigh. Thus, a light valve for display can be realized by aone-dimensional array structure.

In addition, in the embodiment, by assigning a plurality of opticalswitching devices to a pixel, each of the optical switching devices canbe driven independently. Therefore, in an image display, the gradationdisplay of an image can be realized not only by a method of timedivision but also a method of area division.

Image Display

FIG. 22 shows the configuration of a projection display as an imagedisplay using the optical switching device 100. An example of usingreflection light P₃ from the optical switching devices 100A to 100D willbe described here.

The projection display includes light source 200 a, 200 b, and 200 ctaking the forms of lasers of red (R), green (G), and blue (B), opticalswitching device arrays 201 a, 201 b, and 201 c provided incorrespondence with the light sources, dichroic mirrors 202 a, 202 b,and 202 c, a projection lens 203, a galvanometer mirror 204 as anuniaxial scanner, and a projection screen 205. The three primary colorsare not limited to red, green, and blue but may be cyan, magenta, andyellow. Each of the switching device arrays 201 a, 201 b, and 201 c isobtained by one-dimensionally arranging a plurality of necessary pixels,for example, 1000 pixels in a direction perpendicular to the drawingsheet, and functions as a light valve.

In the projection display, light emitted from the light sources 200 a,200 b, and 200 c of R, G, and B is incident on the light switchingdevice arrays 201 a, 201 b, and 201 c. Preferably, the incident angle isset to be zero so that light is incident perpendicularly. The reflectionlight P₃ from the optical switching devices is condensed to theprojection lens 203 by the dichroic mirrors 202 a, 202 b, and 202 c. Thelight condensed by the projection lens 203 is scanned by thegalvanometer mirror 204 and is projected as a two-dimensional image onthe projection screen 205.

In the projection display as described above, a plurality of opticalswitching devices are arranged one-dimensionally, irradiated with lightof RGB, and switched light is scanned by the uniaxial scanner, therebyenabling a two-dimensional image to be displayed.

In the embodiment, the reflectance in a low reflection mode can be setto 0.1% or lower, and that in a high reflection mode can be set to 70%or higher. Consequently, an image can be displayed at a high constant ofabout 1000 to 1, and a characteristic at a position light is incident onan device perpendicularly can be demonstrated, so that it is unnecessaryto consider polarization of light and the like at the time of assemblingan optical system, and the configuration is simple.

The invention has been described by the foregoing embodiments but is notlimited to the foregoing embodiments and modifications and can bevariously modified. For example, in the embodiment, the display of theconfiguration of scanning the light valves in a one-dimensional array byusing a laser as a light source has been described. As shown in FIG. 23,a configuration of displaying an image on the projection screen 208 byemitting light from a white light source 207 to an optical switchingdevice 206 in which devices are arranged two-dimensionally can be alsoemployed.

In the foregoing embodiment, the example of using the glass substrate asa substrate has been described. Alternately, as shown in FIG. 24, adirect-view-type, paper-state display using a flexible substrate 209having a thickness of, for example, 2 mm or less may be used.

Further, in the embodiment, the example of using the optical multilayerstructure of the invention for the display has been described. It isalso possible to apply the optical multilayer structure of the inventionto various devices other than the display, such as an optical printer.For example, the optical multilayer structure is used for an opticalprinter to form an image onto a photosensitive drum.

Second Embodiment

Each of FIGS. 25 and 26 shows a basic configuration of an opticalmultilayer structure 2 according to a second embodiment of theinvention. FIG. 25 shows a state where a gap portion 53 which will bedescribed hereinlater exists in the optical multilayer structure 2, andFIG. 26 shows a state where there is no gap portion in the opticalmultilayer structure 2.

The optical multilayer structure 2 is constructed by stacking, on asubstrate 50 made of, for example, a metal, a first transparent layer 51made of a material having a high refractive index, a second transparentlayer 52 made of a material having a low refractive index, a gap portion53 having a changeable size capable of causing an optical interferencephenomenon, and a third transparent layer 54 made of a material having ahigh refractive index.

The position of the gap portion 53 is not limited to the example (theposition between the second and third transparent layers 52 and 54) butthe gap portion 53 may be provided between the substrate 50 and thefirst transparent layer 51 or between the first and second transparentlayers 51 and 52. The reflection characteristic on a high reflectanceside varies according to the position of the gap portion.

In the embodiment, a refractive index nm of the substrate 50 and anextinction coefficient k_(m) satisfy the relations of the followingexpressions (9) and (10) but do not satisfy the relations of thefollowing expressions (11) and (12).0.33≦n _(m)≦17.45  (9)k _(m)≦{square root}{square root over ( )}73.27−(n _(m)−8.89)²  (10)1≦n _(m)≦5.76  (11)k _(m)≦{square root}{square root over ( )}5.66−(n _(m)−3.38)²  (12)

As specific materials of the substrate 50, metals such as chromium (Cr)and titanium (Ti) can be mentioned. Other than those materials, a metalnitride such as titanium nitride (TiN_(x)), a semiconductor such asgermanium (Ge), or an opaque oxide such as chromium oxide (CrO) may beused.

The “material having a high refractive index” of the first and thirdtransparent layers 51 and 54 has a refractive index n of 2.0 or higher.For example, TiO₂ (n=2.4), Nb₂O₅ (n=2.1), and Ta₂O₅ (n=2.1) can bementioned. On the other hand, the “material having a low refractiveindex” of the second transparent layer 52 has a refractive index n lowerthan 2.0. For example, MgF₂ (n=1.38), SiO₂ (n=1.46), and Al₂O₃ (n=1.67)can be mentioned. The second transparent layer 52 having a lowrefractive index may be an air layer (n=1.0) or the like similar to thegap portion 53 which will be described hereinlater. In this case, thesize d₂ of the second transparent layer (air layer) is fixed.

Although the lowest layer is the substrate 50 in this example, it may bereplaced by a layer or portion such as a Cr film of, for example, 100 nmwhich absorbs incident light so that transmission light becomessubstantially zero.

Each of the optical film thicknesses d₁ and d₂ of the first and secondtransparent layers 51 and 52 is equal to or smaller than “λ/2” (λdenotes a design wavelength of incident). The optical film thickness d₃of the third transparent layer 54 is “λ/4”. The film thicknesses d₁ andd₂ are not strictly equal to “λ/2” and “λ/4”, respectively, but may beapproximately equal to the values for the following reason. For example,when the film thickness d₁ of the first transparent layer 51 becomesthicker than λ/2, it can be compensated by, for example, reducing thethickness of the second transparent layer 52. A slight deviation from anideal refractive index in the expressions (8) to (11) may be compensatedby adjusting the film thickness. In the specification, the expressions“λ/2” and “λ/4” include values “approximately λ/2” and “approximatelyλ/2” and “approximately λ/4”.

Each of the second and third transparent layers 52 and 54 may take theform of a composite layer constructed by two or more layers havingoptical characteristics different from each other. In this case, theoptical characteristics (optical admittance) of the composite layer haveto be equivalent to those in the case of a single layer.

The gap portion 53 is set to that its optical size (the interval betweenthe second and third layers 52 and 53) can be varied by driving meansdescribed hereinlater. A medium filling the gap portion 53 may be gas orliquid as long as it is transparent. Examples of the gas are air (havinga refractive index n_(D)=1.0 with respect to sodium D ray (589.3 nm)),nitrogen (N₂) (n_(D)=1.0) and the like. Examples of the liquid are water(n_(D)=1.333) silicone oil (n_(D)=1.4 to 1.7), ethyl alcohol(n_(D)=1.3618), glycerin (n_(D)=1.4730), diiodomethane (n_(D)=1.737),and the like. The gap portion 12 may be in a vacuum state.

The optical size d₄ of the gap portion 53 changes in a binary manner orcontinuously between “an odd multiple of λ/4” and “an even multiple ofλ/4 (including 0)”. Accordingly, the amount of reflection, transmission,or absorption of incident light changes in a binary manner orcontinuously. Like the case of the thickness of the third transparentlayer 54 in FIG. 13, consequently, the expression of “λ/4” includes“approximately λ/4”.

In the optical multilayer structure 2 of the embodiment, by changing theoptical size of the gap portion 53, an amount of reflection of lightincident on the side opposite to the substrate 50. Concretely, theoptical size of the gap layer 53 is changed between an odd multiple ofλ/4 and an even multiple of λ/4 (including 0) (for example, between“λ/4” and “0”) in a binary manner or continuously, an amount ofreflection, transmission, or absorption of incident light is changed.

By referring to FIGS. 5A and 5B, 6A and 6B, and FIG. 27, the meaning ofEquations (8) to (II) will be described.

In the optical multilayer structure of the first embodiment, asdescribed by using FIGS. 5A and 5B, and 6A and 6B, when the optical filmthickness of the air layer as the gap portion 53 is changed from “0” to“λ/4”, the reflectance changes from “0%” to “76%” in the case where thesubstrate is made of a non-metallic metal material such as a glass, thatis when k=0 in the complex index N of refraction=n−i·k.

In contrast, when the substrate is made of an opaque metal material, kis not equal to zero. Consequently, the start point of a locus on anadmittance diagram is (n, −k) on the diagram. When the substrate is madeof, for example, chromium (Cr), as shown in FIG. 27, n is equal to 3.11and k is equal to 4.42 at an incident wavelength μ of 550 nm. Like theforegoing example, in order to obtain an antireflection characteristic,a high refractive index layer (first transparent layer 11) such as TiO₂(n=2.40) is formed at the point (3.11, −4.42) of Cr and a low refractivelayer (second transparent layer 12) made of SiO₂ (n=1.46) or the like isformed on the first transparent layer 51. A composite admittance isconsequently realized at a point (0, 5.76) on a real axis Re. When ahigh refractive index layer (third transparent layer 54) made of TiO₂(n=2.4) or the like is formed with a film thickness of λ/4 on the secondtransparent layer 52, the locus of the composite admittance returns tothe point (0,1) on the real axis, so that no reflection occurs. That is,as shown in FIG. 26, in a state where the gap portion 53 in the opticalmultilayer structure 2 is “0”, incident light is absorbed and noreflection light is generated.

Third Embodiment

In the embodiment, on the substrate 50, the first transparent layer 51made of a material having a high refractive index, the secondtransparent layer 52 made of a material having a low refractive index,and the third transparent layer 54 made of a material having a highrefractive index are sequentially stacked in this order. When thesubstrate 50 is made of an opaque metal material, according to the startpoint of the complex index of refraction, that is, admittance, as shownin FIG. 28 or 29, a first transparent layer 51A made of a materialhaving a low refractive index and a second transparent layer 52A made ofa material having a high refractive index are stacked on the substrate50 in this order.

In the optical multilayer structure 3 shown in FIG. 28, a gap portion53A taking the form of, for example, an air layer is provided betweenthe first transparent layer 51A made of the material having a lowrefractive index and the second transparent layer 52A made of thematerial having a high refractive index. In contrast, in the opticalmultilayer structure 4 shown in FIG. 29, the gap portion 53A taking theform of, for example, an air layer is provided between the substrate 50and the first transparent layer 51A made of the material having a lowrefractive index. The first transparent layer 51A having a lowrefractive index may take the form of the same air layer (n=1.0) as thegap portion 53A. However, different from the gap portion 53A, the sizeof this air layer is fixed.

FIG. 30 is an admittance diagram for explaining a different point indesigning between the optical multilayer structure of the secondembodiment (FIG. 25) and that of the third embodiment (FIGS. 28 and 29).The diagram shows the ranges of starting materials of the substratewhich can be used in the case of the configuration in which TiO₂ (n=2.4)having the highest refractive index is used as the material of the firsttransparent layer 51 and MgF₂ (n=1.38) having the lowest refractiveindex is used as the material of the second transparent layer 52, and inthe case of the configuration in which MgF₂ (n=1.38) is used as thematerial of the first transparent layer 51A and TiO₂ (n=2.4) is used asthe material of the second transparent layer 52A.

In FIG. 30, a first area A indicated by hatch lines which slant to theleft corresponds to the case where the first transparent layer 51Ahaving a low refractive index and the second transparent layer 52Ahaving a high refractive index are formed on the substrate 50 (that is,the optical multilayer structure 3 or 4 in FIG. 28 or 29). Examples ofthe substrate material corresponding to the first area A include carbon(C), silicon (Si), germanium (Ge), and tantalum (Ta). On the other hand,a second area B indicated by hatch lines which slant to the rightcorresponds to the case where the first transparent layer 51 having ahigh refractive index, the second transparent layer 52 having a lowrefractive index, the gap portion 53, and the third transparent layer 54having a high refractive index are formed on the substrate 50 (that is,the optical multilayer structure 2 in FIG. 25). In the second area B,the configuration in which the gap portion 53 is between the substrate50 and the first transparent layer 51 or between the second transparentlayer 52 and the third transparent layer 54. Examples of the substratematerial corresponding to the second area B include, in addition to Crdescribed above, Ti and Nb.

When the refractive index of the substrate 50 is nm and the extinctioncoefficient is k_(m) (zero in the case of a transparent substrate) atthe wavelength λ of incident light, the first area A applied to theoptical multilayer structures 3 and 4 in FIGS. 28 and 29 satisfies theforegoing expressions (11) and (12).

On the other hand, the second area B applied to the optical multilayerstructure 2 of FIG. 25 is an area satisfying the relations of the aboveexpressions (9) and (10), while do not satisfying the relations of theexpressions (11) and (12). In the case of the optical multilayerstructure 3 (FIG. 28) having the configuration of the substrate/lowrefractive index layer/gap portion/high refractive index layer or theoptical multilayer structure 4 (FIG. 29) having the configuration ofsubstrate/gap portion/low refractive index layer/high refractive indexlayer, when the refractive index n is in a range from 1.90 to 5.76, thetransparent substrate having the extinction coefficient k=0 alsosatisfies the relations of the expressions (11) and (12). Consequently,a transparent substrate made of glass, plastic, or the like, other thana metal, can be also applied.

When a material having a high refractive index and a lower (n) value ora material having a low refractive index and a higher (n) value is used,the range becomes narrower. In the case of a material having a valueoutside of the range, although there is a solution, the number of layersincreases.

EXAMPLES

FIG. 31 shows the relation between the wavelength (design wavelength 550nm) of incident light and the reflectance in the case where Cr(n_(m)=3.12, k=4.42) is used as the material of the substrate 50, a TiO₂film (n₁=2.32) is used as the first transparent layer 51, an SiO₂ film(n=1.46) is used as the second transparent layer 52, an air layer(n=1.00) is used as the gap portion 53, and a TiO₂ film is used as thethird transparent layer 54. In this case, characteristic in the casewhere the optical film thickness of the gap portion (air layer) is “0”and “λ/4” are shown.

FIGS. 32A and 32B are admittance diagrams of this case. FIG. 32A showsthe characteristic when the optical film thickness of the gap portion(air layer) is “0” (that is, the characteristic at the time of lowreflection). FIG. 32B shows the characteristic when the optical filmthickness of the gap portion (air layer) is “λ/4” (that is, thecharacteristic at the time of high reflection).

As obviously understood from the characteristic diagram of FIG. 31, inthe optical multilayer structure 2 shown in FIG. 25, when the opticalfilm thickness of the gap portion (air layer) 53 is “λ/4”, a highreflection characteristic is demonstrated for incident light (λ=550 nm).When the optical film thickness of the gap portion 53 is 0, a lowreflection characteristic is demonstrated.

Further, FIGS. 33 and 34 show the state that the reflectancecharacteristic at the time of high reflectance in the optical multilayerstructure 2 in FIG. 25 varies according to the position of the gapportion 53. In FIG. 33, (a) indicates the characteristic in the casewhere the gap portion 53 is provided between the substrate 50 and thefirst transparent layer (high refractive index layer) 51, (b) indicatesthe characteristic in the case where the gap portion 53 is providedbetween the first transparent layer (high refractive index layer) 51 andthe second transparent layer (low refractive index layer) 52, and (c)indicates the characteristic in the case where the gap portion 53 isprovided between the second and third transparent layers 52 and 54(corresponding to the examples of FIGS. 25, 31, and 32). With respect tothe reflectance characteristic at the time of high reflectance, theconfiguration (a) is the best, the second best is the configuration (c),and the configuration (b) follows. (d) demonstrates a reflectancecharacteristic at the time of low reflectance.

FIG. 35 shows the reflection characteristic of incident light (designwavelength of 550 nm) when, in the optical multilayer structure 3 of theconfiguration shown in FIG. 28, tantalum (Ta) (n_(m)=2.46, k=1.90) isused as the material of the substrate 50A, an MgF₂ film (n₁=1.38) isused as the first transparent layer (low refractive index layer) 51A, aTiO₂ film (n=2.32) is used as the second transparent layer (highrefractive index layer) 52A, and an air layer (n=1.00) is used as thegap portion 53A disposed between the first and second transparent layers51A and 52A. In this case, the characteristics in the case where theoptical film thicknesses of the gap portion (air layer) are “0” and“λ/4”.

FIGS. 36A and 36B are admittance diagrams of this case. FIG. 36A showsthe characteristic when the optical film thickness of the gap portion(air layer) is “0” (that is, the characteristic at the time of lowreflection). FIG. 36B shows the characteristic when the optical filmthickness of the gap portion (air layer) is “λ/4” (that is, thecharacteristic at the time of high reflection).

FIG. 37 shows the reflection characteristic of incident light (designwavelength of 550 nm) when, in the optical multilayer structure 3 of theconfiguration shown in FIG. 29, tantalum (Ta) (n_(m)=2.46, k=1.90) isused as the material of the substrate 50A, an MgF₂ film (n₁=1.38) isused as the first transparent layer (low refractive index layer) 51A, aTiO₂ film (n=2.32) is used as the second transparent layer (highrefractive index layer) 52A, and an air layer (n=1.00) is used as thegap portion 53A disposed between the substrate 50A and the firsttransparent layer 51A. In this case as well, the characteristics in thecase where the optical film thicknesses of the gap portion (air layer)are “0” and “λ/4” are shown. FIG. 38 is an admittance diagram when theoptical film thickness is “λ/4”.

As understood that, with each of the configurations of FIGS. 35 and 36,when the optical film thickness of the gap portion (air layer) 53A is“λ/4”, a high reflection characteristic is demonstrated for incidentlight (λ=550 nm). When the optical film thickness of the gap portion 53Ais 0, a low reflection characteristic is demonstrated. Theconfigurations have almost the same characteristics.

In the optical multilayer structures 2 to 4 of the second and thirdembodiments, for example, also in a visible light region of, forexample, 550 nm, modulation of high contrast can be performed. Moreover,since the configuration is simple and a movable portion moving range isat most “λ/2” or “λ/4”, high response can be realized. By using any ofthe optical multilayer structures, a high speed optical switching deviceand a high speed image display can be realized.

Although the gap portion in the optical multilayer structure is a singlelayer in the foregoing embodiment, it can consists of a plurality oflayers, for example, two layers as shown in FIG. 39. Specifically, thefirst transparent layer 51, second transparent layer 52, first gapportion 53, third transparent layer 54, second gap layer 60, and thirdtransparent layer 61 are sequentially stacked on the substrate 10, andthe second transparent layer 53 and the third transparent layer 61 aresupported by, for example, supporting member 62 made of silicon nitride.

In the optical multilayer structure, the second transparent layer 52 asan intermediate layer is displaced in the vertical direction, one of thefirst and second gap portions 53 and 60 is accordingly narrowed, and theother gap portion is widened, thereby changing the reflectioncharacteristic.

Since the driving method of each of the optical multilayer structures 2,3, and 4 according to the second and third embodiments is substantiallythe same as that in the first embodiment, its description will not berepeated.

By any of the optical multilayer structures 2, 3, and 4 according to thesecond and third embodiments as well, in a manner similar to the firstembodiment, with a simple configuration, high-response optical switchingdevice and image display can be realized.

Fourth Embodiment

Each of FIGS. 40 and 41 shows a basic configuration of an opticalmultilayer structure 5 according to a fourth embodiment of theinvention. FIG. 40 shows a state where the gap portion 12 which will bedescribed hereinlater exists in the optical multilayer structure 5, andFIG. 41 shows a state where there is no gap portion in the opticalmultilayer structure 5. In a manner similar to the foregoingembodiments, the optical multilayer structure 5 is also used as,concretely for example, an optical switching device. By arranging aplurality of optical switching devices in a one-dimensional array, animage display can be constructed.

The optical multilayer structure 5 of the embodiment is constructed bystacking, on a substrate 70 made of, for example, a non-metaltransparent material, a first transparent layer 71 in contact with thetransparent substrate 70, a gap portion 72 having a changeable sizecapable of causing an optical interference phenomenon, and a thirdsecond transparent layer 73.

When a refractive index of the transparent substrate 70 is n_(s), arefractive index of the first transparent layer 71 is n₁, and arefractive index of the second transparent layer 73 is n₂, they are setso that the following expressions (13) and (14) are satisfied.Specifically, in the optical multilayer structure 5, with respect to therefractive indices, on the transparent substrate 70, the firsttransparent layer 71 having a refractive index higher than that of thesubstrate, the gap portion 72, and the second transparent layer 73having a refractive index lower than that of the first transparent layer71 are stacked in this order.n _(s) <n ₁, and n ₁ >n ₂  (13)n _(s) =n ₁ /{square root}{square root over ( )}n _(s)  (14)

The reason why the expression (14) is set as a requirement is asfollows.

In FIG. 5A, composite admittance of the optical multilayer structure isY₁=n₁ ²/n_(s), Y₃′=n₂ ²/Y₁=n₂ ²·n_(s)/n₁. To realize such acharacteristic, it is sufficient to set that Y₃′=1.0 (admittance ofair). It is therefore sufficient to satisfy n₂ ²·n_(s)/n₁ ²=1.0, thatis, n₂=n₁/{square root}{square root over ( )}n_(s). When the refractiveindices do not strictly satisfy the relation, it can be compensated byadjusting a film thickness or the like to a some extent.

As specific materials of the substrate 70, for example, a transparentglass substrate or transparent plastic substrate is used. Concretematerials of the first transparent layer 71 include titanium oxide(TiO₂) (n₁=2.4), silicon nitride (Si₃N₄) (n₁=2.0), zinc-oxide (ZnO)(n₁=2.0), niobium oxide (Nb₂O₅) (n₁=2.2), tantalum oxide (Ta₂O₅)(n₁=2.1), silicon oxide (SiO) (n₁=2.0), stannic oxide (SnO₂) (n₁=2.0),and ITO (Indium-Tin Oxide) (n₁=2.0). Concrete materials of the secondtransparent layer 13 include silicon oxide (SiO₂) (n₂=1.46), bismuthoxide (Bi₂O₃) (n₂=1.91), magnesium fluoride (MgF₂) (n₂=1.38), andalumina (Al₂O₃) (n₂=1.67).

Each of the optical film thicknesses d₁ and d₂ of the first and secondtransparent layers 71 and 72 is equal to “λ/4” or “an odd multiple ofλ/4” (λ denotes a wavelength of incident light). The film thicknesses d₁and d₂ are not strictly equal to “λ/4”, but may be approximately equalto “λ/4” for the following reason. For example, when the film thicknessd₁ of the first transparent layer 71 becomes thicker than λ/4, it can becompensated by, for example, reducing the thickness of the secondtransparent layer 73. Even when the refractive index is deviated fromthe expression (14) more or less, it may be adjusted by the filmthickness. In this case, each of the film thicknesses d₁ and d₂ isslightly deviated from λ/4 in a manner similar to the other embodiments.Therefore, the expression “λ/4” includes “approximately λ/4”.

Each of the first and third layers 71 and 73 may take the form of acomposite layer constructed by two or more layers having opticalcharacteristics different from each other. In this case, the opticalcharacteristics (optical admittance) of the composite layer have to beequivalent to those in the case of a single layer.

The gap portion 72 is set to that its optical size (the interval betweenthe first layer 71 and the second layer 73) can be varied by drivingmeans described hereinlater. A medium filling the gap portion 72 may begas or liquid as long as it is transparent. Examples of the gas are air(having a refractive index n_(D)=1.0 with respect to sodium D ray (589.3nm)), nitrogen (N₂) (n_(D)=1.0) and the like. Examples of the liquid arewater (n_(D)=1.333) silicone oil (n_(D)=1.4 to 1.7), ethyl alcohol(n_(D)=1.3618), glycerin (n_(D)=1.4730), diiodomethane (n_(D)=1.737),and the like. The gap portion 72 may be in a vacuum state.

The optical size of the gap portion 72 changes in a binary manner orcontinuously between “an odd multiple of λ/4” and “an even multiple ofλ/4 (including 0)”. Accordingly, the amount of reflection ortransmission of incident light changes in a binary manner orcontinuously. Like the case of the first and second layers 71 and 73,even when the optical size is slightly deviated from a multiple of λ/4,it can be compensated by a slight change in the film thickness orrefractive index of the other layer. Consequently, the expression of“λ/4” includes “approximately λ/4”.

In the optical multilayer structure 5, by changing the optical size ofthe gap portion 72, the amount of reflection or transmission of lightentering from either the transparent substrate 70 side or the sideopposite to the transparent substrate 70 is changed. Concretely, theoptical size of the gap portion 72 is changed between an odd multiple ofλ/4 and an even multiple of λ/4 (including 0) (for example, between“λ/4” and “0”) in a binary manner or continuously, an amount ofreflection or transmission of incident light is changed.

EXAMPLE

FIGS. 42A and 42B show the relation between the wavelength (designwavelength of 550 nm) of incident light and the reflection when, in theoptical multilayer structure 5, a glass substrate (n_(s)=1.52) is usedas the transparent substrate 10, a TiO₂ film (n₁=2.32) is used as thefirst transparent layer 71, an air layer (n_(D)=1.00) is used as the gaplayer 72, and a Bi₂O₃ film (refractive index n₂=1.92) is used as thesecond transparent layer 73. In this case, FIG. 42A shows thecharacteristics in the case where the optical film thicknesses of thegap portion (air layer) are “λ/2” (physical thickness=275 nm) and “λ/4”(137.5 nm). FIG. 42B shows the characteristics in the case where theoptical film thicknesses of the gap portion (air layer) are “0” and“λ/4”.

FIGS. 43A to 43C are optical admittance diagrams. FIG. 43A shows thecharacteristic in the case where the optical film thickness of the gapportion (air layer) is “λ/2”. FIG. 43B shows the characteristic in thecase where the optical film thickness of the gap portion (air layer) is“λ/4”. FIG. 43C shows the characteristic in the case where the opticalfilm thickness of the gap portion (air layer) is “0”.

As understood from the characteristics diagrams shown in FIGS. 42A and42B, in the optical multilayer structure 5, when the optical filmthickness of the gap portion (air layer) 72 is “λ/2”, a low reflectioncharacteristic is demonstrated for incident light (wavelength λ). Whenthe optical film thickness of the gap portion 72 is “λ/4”, a highreflection characteristic is demonstrated. When the optical filmthickness of the gap portion 72 is 0, a low reflection characteristic isdemonstrated. That is, when the optical film thickness of the gapportion 72 is switched between an odd multiple of “λ/4” and an evenmultiple of “λ/4” including 0, the high reflectance characteristic andthe low reflectance characteristic are alternately demonstrated. Even inthe case of the low reflectance characteristic as well, when the opticalfilm thickness is “λ/2”, a V-coat reflectance characteristic isdemonstrated at a specific wavelength (550 nm). When the optical filmthickness becomes “0”, the V-shape characteristic becomes gentle andthan close to flat, so that the band of reflectance of 0% becomes wider.

In the embodiment, also in a visible light region of, for example, 550nm, modulation of high contrast can be performed. Moreover, since theconfiguration is simple and a movable portion moving range is at most“λ/2”, high response can be realized. By using the optical multilayerstructure 5, a high speed optical switching device and a high speedimage display can be realized.

Fifth Embodiment

By referring to FIGS. 44 and 45, a fifth embodiment of the inventionwill now be described. In the fifth embodiment, the reflectioncharacteristic can be almost evenly changed in a wide wavelength rangehaving a predetermined width (flat range).

An optical multilayer structure 6 is constructed by stacking, on atransparent substrate 80 made of, for example, a non-metal transparentmaterial, a first transparent layer 81 in contact with the transparentsubstrate 80, a second transparent layer 82, a gap portion 83 having achangeable size capable of causing an optical interference phenomenon, athird transparent layer 84, and a fourth transparent layer 85. FIG. 44shows a state where the gap portion 83 which will be describedhereinlater in the optical multilayer structure 6 exists. FIG. 45 showsa state where there is no gap portion in the optical multilayerstructure 6.

In the embodiment, when refractive indices of the substrate 80, thefirst transparent layer, the second transparent layer, the thirdtransparent layer, and the fourth transparent layer are n_(s), n₁, n₂,n₃, and n₄, they are set so as to satisfy the relations of the followingexpression (15).n _(s) <n ₁ <n ₂ ≈n ₃, and n ₄ <n ₁  (15)

As specific materials, for example, a transparent glass substrate ortransparent plastic substrate is used as the transparent substrate 80,an Al₂O₃ film (n₁=1.67) is used as the first transparent layer 81, aTiO₂ film (n₂=2.4) is used as the second transparent layer 82, a TiO₂film (n₃=2.4) is used as the third transparent layer 84, and an MgF₂film (n₄=1.38) is used as the fourth transparent layer 85. Each ofoptical film thicknesses n₁d₁, n₂d₂, n₃d₃, and n₄d₄ of the first tofourth transparent layers 81 to 85 is either “λ/4” or “an odd multipleof λ/4” (λ denotes a wavelength of incident light).

Each of the first to fourth layers 81 to 85 may take the form of acomposite layer constructed by two or more layers having opticalcharacteristics different from each other. In this case, the opticalcharacteristic (optical admittance) in the composite layer has to havecharacteristics equivalent to those in the case of a single layer.

The gap portion 83 is set so that its size (the interval between thesecond transparent layer 82 and the third transparent layer 84) can bevaried by driving means described hereinlater in a manner similar to thegap portion 72 in the first embodiment. A medium filling the gap portion83 is similar to that in the case of the gap portion 72.

In the optical multilayer structure 6, Expression (15) is satisfied.Consequently, the reflection characteristic can be obtained in a widerange and, by changing the size of the gap portion 83, the amount ofreflection or transmission of light incident on the transparentsubstrate 80 side or the side opposite to the transparent substrate 80is changed. More concretely, like the fourth embodiment, by changing theoptical size of the gap portion 83 in a binary manner or continuouslybetween “an odd multiple of λ/4” and “an even multiple of λ/4 (including0)”, the amount of reflection, transmission, or absorption of incidentlight is changed in a binary manner or continuously.

FIG. 46 is a characteristic diagram showing the relation between thewavelength (design wavelength) of incident light and reflectance in thecase where a glass substrate (n_(s)=1.52) is used as the transparentsubstrate 80, a composite layer of a TiO₂ film and an MgF₂ (magnesiumfluoride) film (complex index of refraction n₁=1.7 corresponding to acomposite film thickness λ/4) is used as the first transparent layer 81,a TiO₂ film (refractive index n₂=2.32) is used as the second transparentlayer 82, an air layer is used as the gap portion 83, a TiO₂ film(refractive index n₃=2.32) is used as the third transparent layer 84,and an MgF₂ film (refractive index n₄=1.38) is used as the fourthtransparent layer 85. FIG. 46 shows both the characteristic in the casewhere the optical film thickness of the gap portion (air layer) 83 is“λ/4” and that in the case where the optical film thickness of the gapportion (air layer) 83 is “0”.

FIGS. 47A and 47B are optical admittance diagrams. FIG. 47A shows thecase where the optical film thickness of the gap portion (air layer) 83is “0” and FIG. 47B shows the case where the optical film thickness ofthe gap portion (air layer) 83 is “λ/4”.

As obviously understood from the characteristic diagram of FIG. 46, inthe optical multilayer structure 6 of the embodiment, when the opticalfilm thickness of the gap portion (air layer) 83 is “λ/4”, a highreflection characteristic (approximately 60%) is demonstrated in a widerange. When the optical film thickness of the gap portion 83 is “0”, alow reflection characteristic which is flat in a wide range isdemonstrated.

[Modification]

An optical multilayer structure shown in FIG. 48 is formed by stacking ametal film, such as an aluminum (Al) layer 70 a having a thickness of100 nm or more, a TiO₂ film having a thickness of 52.67 nm as the firsttransparent layer 71, and a multilayer film, as the second transparentlayer 73, consisting of a TiO₂ film 73 a having a thickness of 32.29 nm,an SiO₂ film 73 b having a thickness of 114.72 nm, a TiO₂ film 73 chaving a thickness of 53.08 nm, and an SiO₂ film 73 d having a thicknessof 19.53 nm. When the aluminum layer 70 a becomes 100 nm thick orthicker, it hardly transmits light. When an antireflection film isprovided for the non-transmitting film, it means that reflection is zeroand all of light is absorbed by the aluminum layer 70 a. Further,because of the characteristic of aluminum itself, reflectance of about10% in a high reflection state when the characteristic of antireflectiondeteriorates can be easily realized by the smaller number of layers froma design viewpoint.

FIG. 49 shows the result of simulation when the gap portion 72 ischanged in the optical multilayer structure. Incident light enters fromthe side opposite to the aluminum layer 70 a (the side of the SiO₂ film73 d as the uppermost layer). Even when the position of the gap portion72 is changed to the SiO₂ film 73 b which is the second layer from theuppermost layer, similar characteristics can be obtained. FIG. 50 showsa reflection characteristic in this case. The design wavelength is 550nm and the thicknesses of the layers are shown in the diagram.

Although the gap portion 72 in FIGS. 49 and 50 is assumed as air orvacuum having a refractive index of 1.0, a complicated process isnecessary to form it in an experiment. Consequently, an experiment wasconducted by using SiO₂ as a material of a low refractive in place ofthe gap portion. The film configuration is as shown in FIG. 51, and FIG.52 shows the result of the simulation. Measurement results of theactually formed structures are shown in FIGS. 53 and 54. It isunderstood from the drawings that the result of simulation and theresult of actual measurement coincide well with each other. Since thelight is set to enter from the glass substrate side, a measurement valueof reflectance of about 4% on the surface is large. Other than that,controlling a film thickness while optically monitoring the filmthickness during film formation can prevent some deviation.

FIG. 55 shows the reflection characteristic in the case where the gapportion 72 in the optical multilayer structure shown in FIG. 48 is setto 386 nm. FIG. 56 shows the reflection characteristic i the case wherethe gap portion 72 is set to 1485 nm. It is understood that the width ofthe low reflection range in each of FIGS. 55 and 56 is narrower thanthat in the example (the gap portion 72 of 110.46 nm) of FIG. 49. Thatis, the wider the gap portion 72 becomes, the narrower the lowreflection range becomes. The margin at the time of manufacture isaccordingly narrow, so that handling becomes very difficult. The size ofthe gap portion 72 is narrower than 1500 nm, preferably, 500 nm. Withsuch a size, the fabrication is not so difficult.

An optical multilayer structure shown in FIG. 57 is formed by directlystacking a TiO₂ film having a thickness of 40.89 nm as the firsttransparent layer 71, and a multilayer film, as the second transparentlayer 73. The multilayer film consists of a TiO₂ film 73 a having athickness of 32.62 nm, an SiO₂ film 73 b having a thickness of 77.14 nm,a TiO₂ film 73 c having a thickness of 39.40 nm, and an SiO₂ film 73 dhaving a thickness of 163.13 nm. FIG. 58 shows the result of simulationwith a design wavelength of 550 nm. Different from the example of FIG.48, the light absorption film (aluminum layer) is not provided on thetransparent substrate 70, so that the reflectance in the reflection bandis low. However, transmission light passes through the multilayerstructure without being absorbed, thereby enabling the structure itselffrom being heated.

Although the gap portion in the optical multilayer structure is a singlelayer in any of the foregoing embodiments, it can be a multilayer of,for example, two layers as shown in FIG. 59. Specifically, on thetransparent substrate 70, the first transparent layer 71, first gapportion 72, second transparent layer 73, second gap portion 74, andthird transparent layer 75 are formed in accordance with this order, andthe second and third transparent layers 73 are supported by supportingmembers 77 and 76 made of, for example, silicon nitride, respectively.

In the optical multilayer structure, the second transparent layer 73 asan intermediate layer is displaced vertically, one of the first andsecond gap portions 72 and 74 is narrowed, and the other gap portion isaccordingly widened, thereby changing the reflection characteristic.

Since methods of driving the optical multilayer structures 4 and 5 aresimilar to that of the first embodiment, their description will not berepeated.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

1-58. (canceled)
 59. An optical multilayer structure comprising: atransparent substrate made of a non-metallic material; a firsttransparent layer in contact with the transparent substrate; a gapportion having a changeable size capable of causing an opticalinterference phenomenon; and a second transparent layer, the firsttransparent layer, the gap portion, and the second transparent layersbeing stacked in accordance with this order on the transparentsubstrate, wherein when a refractive index of the transparent substrateis n_(s), a refractive index of the first transparent layer is n₁, and arefractive index of the second transparent layer is n₂, the relation ofn_(s)<n₁, and the relation of n₁>n₂ are satisfied.
 60. An opticalmultilayer structure according to claim 59, further comprising drivingmeans for changing an optical size of the gap portion, wherein the sizeof the gap portion is changed by the driving means, thereby changing anamount of reflection or absorption of incident light entered from thetransparent substrate side or a side opposite to the transparentsubstrate.
 61. An optical multilayer structure according to claim 60,wherein an optical thickness of the first and second transparent layersis λ/4 or an odd multiple of λ/4 (λ: wavelength of incident light). 62.An optical multilayer structure according to claim 61, wherein theoptical size of the gap portion is changed by the driving means in abinary manner or continuously between an odd multiple of λ/4 and an evenmultiple of λ/4 (including 0), thereby changing the amount of reflectionor transmission of incident light in a binary manner or continuously.63. An optical multilayer structure according to claim 59, wherein thevalue of n₂ is n₁/{square root}{square root over ( )}n_(s).
 64. Anoptical multilayer structure according to claim 1, wherein at least oneof the first and second transparent layers is a composite layer made oftwo or more layers having optical characteristics different from eachother.
 65. An optical multilayer structure according to claim 64,wherein a part of each of the first and second transparent layers is atransparent conductive layer, and the driving means changes the opticalsize of the gap portion by an electrostatic force generated byapplication of a voltage to the transparent conductive film.
 66. Anoptical multilayer structure according to claim 65, wherein thetransparent conductive film is made of ITO, SnO₂, or ZnO.
 67. An opticalmultilayer structure according to claim 59, wherein the gap portion isfilled with air, transparent gas, or transparent liquid.
 68. An opticalmultilayer structure according to claim 67, wherein the gap portion isfilled with a liquid, and functions as a layer having a low refractiveindex, an intermediate refractive index, or a high refractive index. 69.An optical multilayer structure according to claim 59, wherein the gapportion is in vacuum.
 70. An optical multilayer structure according toclaim 60, wherein the driving means changes an optical size of the gapportion by using a magnetic force. 71-78. (canceled)
 79. An opticalswitching device comprising: an optical multilayer structure having atransparent substrate made of a non-metallic material, a firsttransparent layer, a gap portion having a changeable size capable ofcausing an optical interference phenomenon, and a second transparentlayer, the first transparent layer, the gap portion, and secondtransparent layer being stacked in accordance with this order on thetransparent substrate; and driving means for changing an optical size ofthe gap portion, wherein when a refractive index of the transparentsubstrate is n_(s), a refractive index of the first transparent layer isn₁, and a refractive index of the second transparent layer is n₂, therelation of n_(s)<n₁, and the relation of n₁>n₂ are satisfied. 80.(canceled)
 81. An image display for displaying a two-dimensional imageby irradiating a plurality of optical switching devices arrangedone-dimensionally or two-dimensionally with light, the optical switchingdevice comprising: an optical multilayer structure having a transparentsubstrate made of a non-metallic material, a first transparent layer, agap portion having a changeable size capable of causing an opticalinterference phenomenon, and a second transparent layer, the firsttransparent layer, the gap portion, and the second transparent layerbeing stacked in accordance with this order on the transparentsubstrate; and driving means for changing an optical size of the gapportion, wherein when a refractive index of the transparent substrate isn_(s), a refractive index of the first transparent layer is n₁, and arefractive index of the second transparent layer is n₂, the relation ofn_(s)<n₁ and the relation of n₁>n₂ are satisfied.
 82. (canceled)